Pipe conduits and flexible tubes of polymers are known and are being used more and more frequently for cooling and brake fluids, as well as for fuel lines. In particular, fuel lines of polyamide, especially polyamide 11 or polyamide 12, belong to the state of the art which is discussed, for example, in "Kunststoffe und Elastomers in der Praxis" (Plastics and Elastomers in Practice), published by W. Kohlhammer, Stuttgart-Berlin-Koln-Mainz (1985).
A disadvantage of these polymer pipe conduits is that permeability of the usual fuels through their walls is undesirably high, particularly when the environmental protection and safety concerns that have arisen in recent years, are considered. Another disadvantage of such pipe conduits resides in the high absorption capacity of the polymers for some components of fuels; this leads to swelling and hence to changes in length of the conduits.
For these reasons, development work has been done to improve the so-called monopipes consisting of a single homogeneous layer of polyamide 11 or polyamide 12. One possibility is conversion to polymer multilayer pipes with special barrier layers of polymers.
From the patents DE 35 10 395 C2, DE 38 27 092 C1 and EP 428 834 A2, fuel lines of ethylene/vinyl alcohol copolymers as barrier layers are known. However, ethylene/vinyl alcohol copolymers having the desired barrier properties are extremely brittle and have very low elongations at break. For this reason, these polymers can be used only in small layer thicknesses. Moreover, when being processed by extrusion, specifically by coextrusion with thermoplastic polymers, these polymers are not thermostable at processing temperatures over 200.degree. C. At these temperatures, crosslinkage of the polymer occurs, leading to gel particle formation. In foils, for which these ethylene/vinyl alcohol copolymers were developed, gel particle formation can be recognized immediately and leads to rejection for optical and aesthetic reasons. In thick-walled pipes with dyed polymers, gel particles are not visible, but diminish quality; thus, extremely stringent quality assurance requirements are necessary.
DE 38 21 723 C1 proposes a three-layer pipe with a continuous interior coating of polyolefin and external protective layers of polyamide. From the literature and permeability measurements, it is known that polyolefins form a good barrier against alcohols, and polyamides are a good barrier against hydrocarbons; however, fuels for motor vehicles often are mixtures of hydrocarbons and alcohols.
As is indicated by FIG. 1, the barrier values of the single polymer components cannot be applied to fuel blends or fuel mixtures and the permeability problem can therefore not be solved satisfactorily by a multilayer construction in which individual polymers have a good barrier effect against particular fuel components. Developments are also known wherein fluorine polymers are used as the barrier layer. However, these polymers are not only very expensive, but are difficult to process and to dispose of; moreover, they show little composite bonding in coextrusion processing.